Moisture-indicating instrument for wood



Nov. 6, 1928.

M. E. DUNLAP. MOISTURE I NDIcAfrING INSTUMENT FOR woon 'Filed Feb. '4, 1928 IN VEN TOR.

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A TTORNE Patented Nov. 6, 192s. v

UNITED" 4siufrrzs' PATENT oFFlcE.

MATTHEW ELBRIDG DUN LAP, 0F MADISON, WISCONSIN, ASSIGNOR T0 THE GOVEEN KENT AND THE PEOPLE 0F THE-UNITED S'lA'JZIslS OF AMERICA.

MOISTURE-INDICATING INSTRUMENT FDR Application filed February 4, 192.8. `Serial No. 251,904.

(GRANTED UNDER THE ACT OFMARCH 3, 1883, AS AMENDED APRIL 30, 1928; 370 0f G. 757.)

This application is made underact of March 3, 1883, chapter 143 (22 Stat. 625) and the invention herein described and claimed may be used by the Government of the United States or any of its officers or employees in the prosecution of work for the Government or b any other' person in the United States wit iout the payment to me of any royalty thereon.

My invention relates to a new and improved moisture indicating instrument ticularly for wood, although with slight modification it may be used for air, textiles, grain, cereals, paper and sopforth, which is inexpensive, accurate and easily used.

The moisture content of hygroscopic sub.

stances in an air dry condition iiuctuates with prevailing humidity conditions. Thus a low relative humidit willreduce the moisture content and a. hlgh humidity will increase the moisture content. By controllin the humidity the moisture content of woo and other hygros'copic substances may be regulated. Conversely, if wood is placed in a small tight container the humidity in the container will be controlled b the wood. Further, if a hole 1s drilled in a. piece of wood and vclosed tightly the humidity obtaining within the opening will correspond to the moisture contentj of the wood whatn ever that maI be if the wood is in an 'air dry condition or elow the ber saturation point. This principle is made use ot in the develop ment of the instrument about to be described.

lt is important for woodworkers to lmow the moisture content ot the wood they are usin in order to guard against defects arising rom shrinking and swelling during and after fabrication. The common-method of determining moisture content by 'ovendrying is cumbersome, expensive and requires a considerable time to make the measurements. A. carefully regulated oven is necessary as well as an accurate scale. in the oven method, moisture discs, which consist of thin pieces of wood cut out of'the wood are weighed, oven dried for about a day and reweighed. The difference between the oven dry weight and the original weight is assumed to be `the moisture content and is usually expressed as a percentage of the oven dry Weight. The difference in weight is not moisture in always entirely due to moisture differences, however, since other volatile materials such as 011s may be driven from the Wood by the heat. This is a source of considerable inaccuracyfin some cases.

It is the purpose ofthe new device to provide a means of measuring the moisture content in the course of a few minutes with an` inexpensive instrument that `can, be carried 1n the pocket. Furthermore the measurements may be made without injuring the board materially. A hole l@ inch in diameter may be drilled on the back of the board or edge of the piece for the insertion of the instrument;l The instrument is provided with f a direct reading scale so that no tables or curves are necessary to interpret the results.

This instrument may be used to measure the-moisture content of other materials, for instance, it may be provided with a flattened perforated shield so that it may be inserted `between sheets of paper, with a sharp pointed perforated shield it may be inserted in bales of hygroscopic material such as cottonl or wool or it may beprovided with a fine vscreen shield :for burying in grain, cereals or our. in each case a special calibration would be necessary. f 4

The device as applied to the measurement of moisture in wood may' be best described by referring to the. attached drawing, in

whichf-Figure Il is a front vertical view of the instrument shown linserte-d in ablock of wood; Figure 2 is a side View.; Figure 3 is a vertical section on the center line; Figure' t is. a top view; and Figure 5 is the bottom` view.V

Similar numerals refer to similar `parts throughout the several views.

The 'instrument consists of a glass capillary tube( l) at the base of which is attached a bag (2) made from a very thin hygroscopic membrane derived from the intestines of cattle and known as bottle capping stock or goldbeaters skin, or from other suitable sources such as iilms of cellulose acetate.4 The bag is secured in place by a suitable wrapping of thread (3) and l ed with the proper amount of mercury 4). At the upe'r end of the capillary tu e is a mercury ook which consists of a chamois skin ad (5) and a'viscose cap (6). The chamois s in i pad is placed over the top of the capillary and the viscose cap in a wet condition is slipped over it and permitted to dry and in so doing it shrinks tightly over the end of the tube holding it securely in place. A few needle pricks are made in the top of the cap (6) to permit the passage of air. This 'lock (6) prevents the formation of pressure in the capillary as the mercury rises or falls in the column. The chamois skin pad provides a satisfactory barrier to prevent the leakage of mercury during shipment or in handling.

The capillary tubing is supported on a channel frame (7) and held by means of clamps (8) held in place by lscrews A scale (10) is provided which shows directly the moisture content of the wood. The capillary tubing (l) is fastened to the upper part of the instrument by means of the stuffing box (11). The bulb is protected by means of a base and housing (12) which provides a smooth surface so that the instrument may be easily but snugly inserted in the wood sol as to close the opening; The housing is further provided with Ventilating holes (13) which permit the air to circulate about the bulb of the instrument. The upper portion of the instrument is fastened to the base by means of screws 14). A cap may be provided, if desired, to cover the bulb of the instrument when it is not in use.

The instrument is used by drilling a hole of the proper diameter (15) into but not through the wood (16) to be tested. The' instrument is inserted so that the shoulder of base (12) is tightly pressed against the wood and the bearing surface below the shoulder is perfectly tight so that there will be no air passage between the inside and outside of the hole. rlhe closing of the hole will cause the humidity of the air in the hole to app-roach equilibrium with the'moisture content ofthe Wood whatever that may be. This hmidity or moisture in the space causes the thin membrane (2) to contract or expand forcing the mercury up or down the column. lf the moisture content is high the bag will enlarge and the mercury will drop by the force of gravity. If the membrane dries out it will force the column upward in a like manner. A period of 5 or 1() minutes is required for the atmosphere in the hole and the mercury bag to adjust themselves to the moisture con,- tent of the wood.

The instrument is calibrated by placing an arbitrary scale along the capillary tube, inserting the instrument in holes in small blocks of non-resinous Wood at diderent moisture contents covering a range from 5 to 25 per cent, allowing it to stand 10 minutes in each hole and the height of the mercury column measured With the arbitrary scale. After the readings have been taken the blocks are weighed, oven dried and reweighed to obtain the loss in weight or moisture. content is then expressed as a percentage of the oven dry weight. These points are plotted on suitable coordinate paper and a smooth curve drawn through them. From this curve the important points may be plotted on the instrument scale and thus made direct reading.

ln designing the instrument the eHect of temperature on the height of the mercury in the glass tube has been eliminated by using a suitable combination of size of bulb, quantity of mercury and diameter of bore. As an ex ample, l have found that this is accomplished in an instrument having a bulb containing 2 grams of mercury and a bore of .25 millimeters. The. instrument can be used through a wide range in temperature without seriously affecting it accuracy.

Having thus described my invention, l claim:

1. An instrument of the character described, a capillary tube, a hygroscoplc membrane capable of shrinking and swelling and containing mercury, attached to one end of the capillary tube, a chamois skin pad at the other end of the capillary tube, a viscose cap adapted to hold said pad 1n place on the capillary tube, a scale adjacent said tube, a support adapted to hermetically engage the capillary tube and the object to be tested, and a housing for said membrane on said support.

2. An instrument of the character described, a capillary tube, a bag of hygroscopic material, and containing mercury, the bag being capable of shrinking and swelling with changes of the humidity of the atmosphere surrounding the bag, said bag being attached to one end of the tube, the relative dimensional characteristics of the bore insa-id tube and the bag being such that the mercury does not rise nor fall in said tube due to temperature changes, a chamois skin pad at the other end of the capillary tube, a viscose cap adapted to hold said pad in place on the capillary tube and thus prevent loss of mercury, a scale adjacent said tube, a support adapted to hermetica-lly engage the capillary tube and the object to be tested, and an apertured housing for said bag on said support.

In testimony whereof I alix my signature.

MATTHEW ELBRIDGE DUNLAP.

The moisture 

